Could the "dormant" form of a vessel also have an effect? Right now, I am imaging a thief/spy character whose vessel switches between the dormant form of a key and the active form of a dagger. And that while it is a key, it shapes itself to any lock and basically if there is a magical lock, if the wielder of the key has more potent magic than the creator of the lock (at the time he made the lock), he can also break magical locks. In theory, the key could be a third form, however I think it would be better if it just had the two.